At the heart of nearly all electrical phenomena is a fundamental property of matter called electric charge. It's an intrinsic quality of particles, much like mass. The particles that make up atoms—protons and electrons—possess this property. Protons are said to have a positive charge, while electrons have a negative charge.

The interaction between these charges follows a simple, universal rule: like charges repel, and opposite charges attract. Two protons will push each other away, as will two electrons. But a proton and an electron will pull toward one another. This simple attraction and repulsion is the basis for everything from static cling to the chemical bonds that hold molecules together.

Electric charge is governed by two key principles. The first is the conservation of charge. In any isolated system, the total amount of electric charge remains constant. You can't create or destroy charge, only move it from one object to another. When you rub a balloon on your head, electrons move from your hair to the balloon. Your hair becomes positively charged and the balloon becomes negatively charged, but no new charge is created. The total charge of the hair-balloon system is the same as it was before.

The second principle is the quantization of charge. This means that charge comes in discrete, indivisible units. The smallest amount of free charge that has ever been observed is the charge of a single electron or a single proton. This fundamental unit is called the elementary charge, denoted by the symbol $e$.

Any observable amount of charge, whether on a dust particle or in a lightning bolt, is always an integer multiple of this elementary charge. You can have a charge of $e$, $2e$, or $-5e$, but you will never find an object with a charge of $0.5e$.

Let's revisit the balloon and hair example. Before you rub them together, both are electrically neutral. They have an equal number of protons (positive charges) and electrons (negative charges), so their net charge is zero.

When you rub the rubber balloon against your hair, the friction gives electrons in your hair enough energy to jump over to the balloon. Rubber has a stronger affinity for electrons than hair does, so the electrons tend to stay on the balloon.

The result? Your hair now has more protons than electrons, giving it a net positive charge. The balloon has gained those extra electrons, giving it a net negative charge. Since opposites attract, your positively charged hair will lift up and stick to the negatively charged balloon. This everyday occurrence is a perfect demonstration of charge conservation and the powerful forces between charged objects.

Ready to check your understanding of electric charge?

These core ideas—the existence of two charge types, their interactions, and the laws of conservation and quantization—form the foundation of electricity and magnetism.